• A database of chondrite analyses including platinum group elements, Ni, Co, Au, and Cr: Implications for the identification of chondritic projectiles

      Tagle, R.; Berlin, J. (The Meteoritical Society, 2008-01-01)
      Siderophile elements have been used to constrain projectile compositions in terrestrial and lunar impact melt rocks. To obtain a better knowledge of compositional differences between potential chondritic projectile types, meteorite analyses of the elements Ru, Rh, Pd, Os, Ir, Pt, Cr, Co, Ni, and Au were gathered into a database. The presented compilation comprises 806 analyses of 278 chondrites including new ICP-MS analyses of Allende and two ordinary chondrites. Each data set was evaluated by comparing element ratios of meteorites from the same chondrite group. Characteristic element abundances and ratios were determined for each group. Features observed in the element abundance patterns can be linked directly to the presence of certain components, such as the abundance of refractory elements Os, Ir, and Ru correlating with the occurrence of refractory inclusions in CV, CO, CK, and CM chondrites. The refined characteristic element ratios appear to be representative not only for meteorites, but also for related asteroidal bodies. Chondrite element ratios were compared to previously published values from impact melt rocks of the Popigai and Morokweng impact structures confirming that an identification of the specific type of projectile (L and LL chondrite, respectively) is possible. The assessment for Morokweng is supported by the recent discovery of an LL chondrite fragment in the impact melt rocks. Ultimately, the database provides valuable information for understanding processes in the solar nebula as they are recorded in chondrites. A new type of complementarity between element patterns of CK and EH chondrites is suggested to be the result of condensation, redox, and transportation processes in the solar nebula.
    • Acquisition of shock remanent magnetization for demagnetized samples in a weak magnetic field (7 μT) by shock pressures 5–20 GPa without plasma-induced magnetization

      Funaki, M.; Syono, Y. (The Meteoritical Society, 2008-01-01)
      Demagnetized samples of cobalt precipitates in a copper matrix were shocked to 5, 10, and 20 GPa in a weak magnetic field of 7.7 micro-T to elucidate the origins of the natural remanent magnetization of meteorites and the magnetic anomalies of impact craters on the moon and Mars. The samples placed in the target acquired shock remanent magnetization (SRM) whose intensity increased up to 21.3 times compared with the demagnetized state, but SRM intensity and shock intensity were not correlated. The SRM direction was in most cases approximately perpendicular to the shock direction. The samples placed 4.8 mm from the impacted surface did not acquire significant magnetization, suggesting no plasma-induced remanent magnetization (PIRM) up to 20 GPa. When the samples were divided into 8 sub-samples, the SRM intensities of sub-samples increased up to 40 times compared with bulk ones and their directions were scattered. Higher coercive force grains were magnetized perpendicular to the shock direction for shocks of 5 and 10 GPa, but at 20 GPa the directions were less systematically oriented. These results suggest that the proposed plasma-induced magnetization of impactites should be reconsidered
    • Applied focused ion beam techniques for sample preparation of astromaterials for integrated nanoanalysis

      Graham, G. A.; Teslich, N. E.; Kearsley, A. T.; Stadermann, F. J.; Stroud, R. M.; Dai, Z.; Ishii, H. A.; Hutcheon, I. D.; Bajt, S.; Snead, C. J.; et al. (The Meteoritical Society, 2008-01-01)
      Sample preparation is always a critical step in the study of micrometer-sized astromaterials available for study in the laboratory, whether their subsequent analysis is by electron microscopy or secondary ion mass spectrometry. A focused beam of gallium ions has been used to prepare electron transparent sections from an interplanetary dust particle (IDP), as part of an integrated analysis protocol to maximize the mineralogical, elemental, isotopic, and spectroscopic information extracted from one individual particle. In addition, focused ion beam (FIB) techniques have been employed to extract cometary residue preserved on the rims and walls of microcraters in 1100 series aluminum foils that were wrapped around the sample tray assembly on the Stardust cometary sample collector. Non-ideal surface geometries and inconveniently located regions of interest required creative solutions. These include support pillar construction and relocation of a significant portion of sample to access a region of interest. Serial sectioning, in a manner similar to ultramicrotomy, is a significant development and further demonstrates the unique capabilities of focused ion beam microscopy for sample preparation of astromaterials.
    • Book Review: Cassini at Saturn: Huygens Results, David M. Harland

      Barnes, J. W. (The Meteoritical Society, 2008-01-01)
    • Eros' Rahe Dorsum: Implications for internal structure

      Greenberg, R. (The Meteoritical Society, 2008-01-01)
      An intriguing discovery of the NEAR imaging and laser-ranging experiments was the ridge system known as Rahe Dorsum and its possible relation with global-scale internal structure. The curved path of the ridge over the surface roughly defines a plane cutting through Eros. Another lineament on the other side of Eros, Calisto Fossae, seems to lie nearly on the same plane. The NEAR teams interpret Rahe as the expression of a compressive fault (a plane of weakness), because portions are a scarp, which on Earth would be indicative of horizontal compression, where shear displacement along a dipping fault has thrust the portion of the lithosphere on one side of the fault up relative to the other side. However, given the different geometry of Eros, a scarp may not have the same relationship to underlying structure as it does on Earth. The plane through Eros runs nearly parallel to, and just below, the surface facet adjacent to Rahe Dorsum. The plane then continues lengthwise through the elongated body, a surprising geometry for a plane of weakness on a battered body. Moreover, an assessment of the topography of Rahe Dorsum indicates that it is not consistent with displacement on the Rahe plane. Rather, the topography suggests that Rahe Dorsum results from resistance of the Rahe plane to impact erosion. Such a plane of strength might have formed in Eros parent body by a fluid intrusion (e.g., a dike of partial melt) through undifferentiated material, creating a vein of stronger rock. Albedo, color and near-infrared spectra could be consistent with a distinct material composition and such a history, although the instruments resolution was not adequate for a definitive detection of such a spatially limited component. However the plane of strength formed, such structural reinforcing might have enabled and controlled the elongated irregular shape of Eros, as well as Rahe Dorsum.
    • Geochemical characterization of moldavites from a new locality, the Cheb Basin, Czech Republic

      Řanda, Zdeněk; Mizera, Jiři; Frána, Jaroslav; Kučera, Jan (The Meteoritical Society, 2008-01-01)
      Twenty-three moldavites from a new locality, the Cheb Basin in Western Bohemia, were analyzed by instrumental neutron activation analysis for 45 major and trace elements. Detailed comparison of the Cheb Basin moldavites with moldavites from other substrewn fields in both major and trace element composition shows that the Cheb Basin is a separate substrewn field. The geochemical data obtained are discussed with respect to the source materials and processes leading to formation of moldavites. The data show that three groups of Cheb Basin moldavites exist. Ten samples of group 1 are characterized by the lowest content of Al, Fe, Na, and other elements representing phyllosilicate minerals, and by high Ca + Mg contents related probably to carbonates. They resemble the "poisonous green" moldavites, a subgroup of the Southern Bohemian moldavites. Seven samples of group 2 and 6 samples of group 3 are similar to typical moldavites of the Southern Bohemian substrewn field. These two groups differ from each other mainly in Al contents; with higher contents of Al and the elements associated with phyllosilicate minerals (namely Ba and Sr), group 3 also resembles the Moravian moldavites. Significant positive correlations between K, Ca, Mg, and Mn found in group 2 of the Cheb Basin moldavites and the enrichment in these elements observed generally in all moldavites, as well as other facts, e.g., high K/Na and K/Rb ratios and the reduced conditions during formation of moldavites, have been attributed to possible contribution to the moldavite source materials of the ash produced by burning of vegetation and soil organic matter present at the pre-impact area.
    • Geochemistry and origin of metal, olivine clasts, and matrix in the Dong Ujimqin Qi mesosiderite

      Kong, P.; Su, W.; Li, X.; Spettel, B.; Palme, H.; Tao, K. (The Meteoritical Society, 2008-01-01)
      The Dong Ujimqin Qi mesosiderite is the first recorded fall of a stony-iron meteorite in China. According to silicate textures and metal composition, this meteorite is classified as a member of subgroup IB. Instrumental neutron activation analyses (INAA) of metals show that the matrix metal has lower concentrations of Os, Ir, Re, and Pt, but higher concentrations of Ni and Au than the 7.5 cm metal nodule present in the meteorite. We attribute these compositional differences to fractional crystallization of molten metal. Studies of olivine clasts show that FeO contents are uniform in individual olivine crystals but are variable for different olivine clasts. Although concentrations of rare earth elements (REEs) change within olivine clasts, they all exhibit a vee-shaped pattern relative to CI chondrites. The relatively high concentrations of REEs in olivine and the shape of REE patterns require a liquid high in REEs and especially in light REEs. As such a liquid was absent from the region where basaltic and gabbroic clasts formed, mesosiderite olivine must have formed in a part of the differentiated asteroid that is different from the location where other mesosiderite silicate clasts formed.
    • Geochemistry of Darwin glass and target rocks from Darwin crater, Tasmania, Australia

      Howard, K. T. (The Meteoritical Society, 2008-01-01)
      Darwin glass formed about 800,000 years ago in western Tasmania, Australia. Target rocks at Darwin crater are quartzites and slates (Siluro-Devonian, Eldon Group). Analyses show 2 groups of glass, Average group 1 is composed of: SiO2 (85%), Al2O3 (7.3%), TiO2 (0.05%), FeO (2.2%), MgO (0.9%), and K2O (1.8%). Group 2 has lower average SiO2 (81.1%) and higher average Al2O3 (8.2%). Group 2 is enriched in FeO (+1.5%), MgO (+1.3%) and Ni, Co, and Cr. Average Ni (416 ppm), Co (31 ppm), and Cr (162 ppm) in group 2 are beyond the range of sedimentary rocks. Glass and target rocks have concordant REE patterns (La/Lu = 5.910; Eu/Eu* = 0.55-0.65) and overlapping trace element abundances. 87Sr/86Sr ratios for the glasses (0.80778-0.81605) fall in the range (0.76481-1.1212) defined by the rock samples. epsilon-Nd results range from -13.57 to -15.86. Nd model ages range from 1.2-1.9 Ga (CHUR) and the glasses (1.2-1.5 Ga) fall within the range defined by the target samples. The 87Sr/86Sr versus 87Rb/86Sr regression age (411 +/- 42 Ma) and initial ratio (0.725 +/- 0.016), and the initial 43Nd/144Nd ratio (0.51153 +/- 0.00011) and regression age (451 +/- 140 Ma) indicate that the glasses have an inherited isotopic signal from the target rocks at Darwin crater. Mixing models using target rock compositions successfully model the glass for all elements except FeO, MgO, Ni, Co, and Cr in group 2. Mixing models using terrestrial ultramafic rocks fail to match the glass compositions and these enrichments may be related to the projectile.
    • Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden, and new evidence for the L-chondrite parent body breakup event

      Heck, P. R.; Schmitz, B.; Baur, H.; Wieler, R. (The Meteoritical Society, 2008-01-01)
      We present noble gas analyses of sediment-dispersed extraterrestrial chromite grains recovered from ~470 Myr old sediments from two quarries (Hällekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhögen quarry, all located in southern Sweden. Both the sediment-dispersed grains and the meteorite Gullhögen 001 were generated in the L-chondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment-dispersed chromite grains have partly been retained during ~470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sediment-dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into an orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting-Robertson (P-R) drag. The fossil meteorite Gullhögen 001 has a low cosmic-ray exposure age of ~0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L-chondrite parent body breakup.
    • The classification of micrometeorites

      Genge, M. J.; Engrand, C.; Gounelle, M.; Taylor, S. (The Meteoritical Society, 2008-01-01)
      Due to their small size, the mineralogical and chemical properties of micrometeorites (MMs) are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites. Identifying which groups of MM are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical, textural, and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies, albeit with minimal bias towards preconceived genetic models. Specifically, the interpretation of MMs requires a rigorous and meaningful classification scheme. At present the classification of MMs is, however, at best ambiguous. A unified petrological-chemical classification scheme is proposed in the current study and is based on observations of several thousand MMs collected from Antarctic ice.
    • The Meteoritical Bulletin, No. 93, 2008 March

      Connolly, H. C.; Smith, C.; Benedix, G.; Folco, L.; Righter, K.; Zipfel, J.; Yamaguchi, A.; Chennaoui Aoudjehane, H. (The Meteoritical Society, 2008-01-01)
      In this edition of the Meteoritical Bulletin, 1443 approved meteorite names with their relevant data are reported, one from a specific location within Africa, 211 from Northwest Africa, 5 from KOREAMET, 598 from the Chinese Antarctic Expedition, 23 from the Americas, 151 from Asia, three from Australia, two from Europe, two from NOVA, and 447 from ANSMET that were not reported in the Meteoritical Bulletin no. 87. Also reported are 4 falls from the Americas. Some highlights of approved meteorites are 10 lunar (including NWA 5000, an 11.528 kg sample), 3 Martian, 4 irons (one from Indonesia), 2 ureilites, 5 mesosiderites, 1 pallasite, 6 brachinites, 3 CV3s, 4 CO3s, 8 CMs, 12 CK3s, and many more. Finally, the Committee on Nomenclature of the Meteoritical Society announces two new names series in North America.